16,877 research outputs found
Exploring the Venus global super-rotation using a comprehensive General Circulation Model
The atmospheric circulation in Venus is well known to exhibit strong
super-rotation. However, the atmospheric mechanisms responsible for the
formation of this super-rotation are still not fully understood. In this work,
we developed a new Venus general circulation model to study the most likely
mechanisms driving the atmosphere to the current observed circulation. Our
model includes a new radiative transfer, convection and suitably adapted
boundary layer schemes and a dynamical core that takes into account the
dependence of the heat capacity at constant pressure with temperature.
The new Venus model is able to simulate a super-rotation phenomenon in the
cloud region quantitatively similar to the one observed. The mechanisms
maintaining the strong winds in the cloud region were found in the model
results to be a combination of zonal mean circulation, thermal tides and
transient waves. In this process, the semi-diurnal tide excited in the upper
clouds has a key contribution in transporting axial angular momentum mainly
from the upper atmosphere towards the cloud region. The magnitude of the
super-rotation in the cloud region is sensitive to various radiative parameters
such as the amount of solar radiative energy absorbed by the surface, which
controls the static stability near the surface. In this work, we also discuss
the main difficulties in representing the flow below the cloud base in Venus
atmospheric models.
Our new radiative scheme is more suitable for 3D Venus climate models than
those used in previous work due to its easy adaptability to different
atmospheric conditions. This flexibility of the model was crucial to explore
the uncertainties in the lower atmospheric conditions and may also be used in
the future to explore, for example, dynamical-radiative-microphysical
feedbacks.Comment: Accepted for publication in Planet. Space Sc
Dark matter cores all the way down
We use high resolution simulations of isolated dwarf galaxies to study the
physics of dark matter cusp-core transformations at the edge of galaxy
formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell
size; ~250 Msun per particle) at which the impact from individual supernovae
explosions can be resolved, becoming insensitive to even large changes in our
numerical 'sub-grid' parameters. We find that our dwarf galaxies give a
remarkable match to the stellar light profile; star formation history;
metallicity distribution function; and star/gas kinematics of isolated dwarf
irregular galaxies. Our key result is that dark matter cores of size comparable
to the stellar half mass radius (r_1/2) always form if star formation proceeds
for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun
and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter
'coreNFW' fitting function that captures this dark matter core growth as a
function of star formation time and the projected stellar half mass radius.
Our results have several implications: (i) we make a strong prediction that
if LCDM is correct, then 'pristine' dark matter cusps will be found either in
systems that have truncated star formation and/or at radii r > r_1/2; (ii)
complete core formation lowers the projected velocity dispersion at r_1/2 by a
factor ~2, which is sufficient to fully explain the 'too big to fail problem';
and (iii) cored dwarfs will be much more susceptible to tides, leading to a
dramatic scouring of the subhalo mass function inside galaxies and groups.Comment: 20 pages; 9 figures; final version to appear in MNRAS including typos
corrected in proo
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Regional and global dust storms on Mars investigated using data assimilation
Extending the Globular Cluster System-Halo Mass Relation to the Lowest Galaxy Masses
High mass galaxies, with halo masses , reveal
a remarkable near-linear relation between their globular cluster (GC) system
mass and their host galaxy halo mass. Extending this relation to the mass range
of dwarf galaxies has been problematic due to the difficulty in measuring
independent halo masses. Here we derive new halo masses based on stellar and HI
gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find
that the GC system mass--halo mass relation for galaxies populated by GCs holds
from halo masses of down to below
, although there is a substantial increase in scatter
towards low masses. In particular, three well-studied ultra diffuse galaxies,
with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass
ratios. We compare our GC system--halo mass relation to the recent model of El
Badry et al., finding that their fiducial model does not reproduce our data in
the low mass regime. This may suggest that GC formation needs to be more
efficient than assumed in their model, or it may be due to the onset of
stochastic GC occupation in low mass halos. Finally, we briefly discuss the
stellar mass-halo mass relation for our low mass galaxies with GCs, and we
suggest some nearby dwarf galaxies for which searches for GCs may be fruitful.Comment: 16 pages, 5 figures, accepted for publication in MNRA
A simplified model of the Martian atmosphere - Part 1: a diagnostic analysis
In this paper we derive a reduced-order approximation to the vertical and horizontal structure of a simplified model of the baroclinically unstable Martian atmosphere. The original model uses the full hydrostatic primitive equations on a sphere, but has only highly simplified schemes to represent the detailed physics of the Martian atmosphere, e.g. forcing towards a plausible zonal mean temperature state using Newtonian cooling. Three different norms are used to monitor energy conversion processes in the model and are then compared. When four vertical modes (the barotropic and first three baroclinic modes) are retained in the reduced-order approximation, the correlation norm captures approximately 90% of the variance, while the kinetic energy and total energy norms capture approximately 83% and 78% of the kinetic and total energy respectively. We show that the leading order Proper Orthogonal Decomposition (POD) modes represent the dominant travelling waves in the baroclinically-unstable, winter hemisphere. In part 2 of our study we will develop a hierarchy of truncated POD-Galerkin expansions of the model equations using up to four vertical modes
Reduced-order models of the Martian atmospheric dynamics
In this paper we explore the possibility of deriving low-dimensional models of the dynamics of the Martian atmosphere. The analysis consists of a Proper Orthogonal Decomposition (POD) of the atmospheric streamfunction after first decomposing the vertical structure with a set of eigenmodes. The vertical modes were obtained from the quasi-geostrophic vertical structure equation. The empirical orthogonal functions (EOFs) were optimized to represent the atmospheric total energy. The total energy was used as the criterion to retain those modes with large energy content and discard the rest. The principal components (PCs) were analysed by means of Fourier analysis, so that the dominant frequencies could be identified. It was possible to observe the strong influence of the diurnal cycle and to identify the motion and vacillation of baroclinic waves
A simplified model of the Martian atmosphere - Part 2: a POD-Galerkin analysis
In Part I of this study Whitehouse et al. (2005) performed a diagnostic analysis of a simplied model of the Martian atmosphere, in which topography was absent and in which heating was modelled as Newtonian relaxation towards a zonally symmetric equilibrium temperature field. There we derived a reduced-order approximation to the vertical and the horizonal structure of the baroclinically unstable Martian atmosphere, retaining only the barotropic mode and the leading order baroclinic modes. Our objectives in Part II of the study are to incorporate these approximations into a Proper Orthogonal Decomposition-Galerkin expansion of the spherical quasi-geostrophic model in order to derive hierarchies of nonlinear ordinary differential equations for the time-varying coefficients of the spatial structures. Two different vertical truncations are considered, as well as three different norms and 3 different Galerkin truncations. We investigate each in turn, using tools from bifurcation theory, to determine which of the systems most closely resembles the data for which the original diagnostics were performed
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